Readers questions on a "Catch 22" situation with boiler flues and advice about
sealant for internal brickwork.

Q Can you help with a “Catch 22” situation regarding boiler flues? A year ago we had to replace our boiler, situated in an inside room, with a Keston condensing model. The old boiler had a flue running to the outside wall using two plastic tubes between the ceiling and upstairs floor.

My plumber used two continuous 4m lengths of the new recommended plastic flue pipe and inserted them through these old flue pipes. When British Gas called to service the boiler, the engineer said I would have to install inspection hatches. I pointed out that the flue pipes had no hidden joints. He said it made no difference.

I phoned the Gas Safe helpline and was told that I would have to provide inspection hatches to check that the pipe did NOT have any joints. My plumber and I are at a loss as to what our responsibilities are.

Surely, if I can provide a statement from my plumber that the correct pipes without joints have been installed and this is kept with all the house paperwork this should be sufficient?

Please can you find a definitive answer with which to answer a British Gas engineer, as this must affect thousands of installations, with big cost implications.

DG, Guildford

A I answered queries about the new gas industry guidelines for flue inspection hatches on July 17, August 14 and September 18, and the questions keep coming thick and fast. The guidelines concern flues running in boxed-in voids or between floors and ceilings, and will require (from January 1 2013) an inspection hatch within 1.5m (5ft) of any concealed joint in a flue system.

I have discussed this matter with heating engineers, and the reasoning behind it does seem sound. Many modern boilers have fan-assisted balanced flues comprising two concentric pipes. The outer pipe is the air intake for the gas or oil combustion process, and the inner pipe is the exhaust for the burnt gases, which include carbon dioxide.

There have been reported cases where sections of flue pipe have not been properly joined, or have come apart after installation, allowing the exhaust gas from the inner pipe to leak into the air-intake outer pipe. The carbon dioxide mixes with the air, causing incomplete combustion of the gas in the boiler and the formation of carbon monoxide, which is a deadly poison. Fatalities have resulted.

This is why the new guidelines require heating engineers to routinely inspect the joints between sections of flue pipe and confirm that they remain sound.

In your case, however, the incoming and outgoing flue pipes are two separate entities, and there are no hidden joints. Therefore no inspection hatches are needed. The British Gas engineer and the Gas Safe helpline person were both mistaken in advising you otherwise. I have a statement from the chair of the “Flues in Voids Communications Group” confirming that a written statement from your heating engineer, quoting his Gas Safe licence number, will suffice as proof that the pipes have no hidden joints.

Get to the source of the damp

Q I read with interest your recent reply (September 18) about the use of a sealant on internal brickwork. I have a very old stone cottage (c1750) where water and damp are penetrating badly on an outside gable end wall, where it gets the full force of any wind and rain. I have been advised to use a masonry waterproofer and I wonder if you would recommend this.

RW, by email

A It’s not possible to advise about dampness in individual buildings without inspecting, of course. But in general, external sealing of solid walls is only a “sticking plaster” remedy. Water-repellent sealants have a limited life and have to be renewed regularly. They can also make matters worse by trapping moisture behind them.

It is unlikely that your cottage has always had this problem, otherwise it would have been dealt with at some time over the past 250 years. I wonder if something might have happened recently to cause it. Water penetration problems through old stone walls often start when the original lime mortar has been covered with cement-based re-pointing. You might wish to investigate whether this is the case with your cottage.